Two questions about dump error and why my computer screen won't work?

So first of when I played WoW I started to get physical memory error(blue screen) but I ignored that few times and then suddenly one day came that windows can't find one fail and can't start. It was like 'file is missing or corrupt' - system32/config/system.

I took that computer back to that place where I bought and told them to fix it(even if that was my problem lol) so they did. It worked few days before it all started again. So, when I put my windows CD and did some things (haha) then system32/config/system didn't come at all. But the physical memory (blue screen) error were still coming. Everytime it was different code, but once I wrote it down... was nv4_mini.sys. If I'm correct it's about video card. So I thought I buy new one.

Today I thought that maybe if I buy new video card and still don't work, it's waste of money so I opened my computer. I changed my 2 ram places. Like I put the second one where the first one was before.

But then... I started my computer ... at first it made some creepy voice.. like few seconds and then screen didn't work. I looked that every wire was connected correctly and then the noise stopped. But screen didn't work. I tryed everything, took video card out and put back... everything but it didn't work. PS! That screen works for my other computer so it's not the screen's problem.

so, my questions.

1.) why my screen won't work?

2.) if I change video card, could the physical memory error(blue screen) stop then? Because I tryed everything what I found on internet... cleaner.. deleted junk etc.

Thanks. (English isn't my first language so sorry of the mistakes lol=